It is well known to use thermally regenerable liquid solvents for extracting the hydrogen sulfide contained in a gas, in particular in a natural gas. Examples of the most commonly used solvents are aqueous amine solutions and some physical solvents such as sulfolane, methanol, N-formyl morpholine, acetyl morpholine, propylene carbonate. Although many solvents likely to extract H2S also allow to extract CO2, some of them however show a selectivity for H2S over CO2 and are therefore used when the amount of CO2 extracted with H2S is to be limited. Examples of the most commonly used H2S selective solvents are methyldiethanolamine (MDEA), diisopropanolamine (DIPA), as well as the sterically encumbered amines and some physical solvents such as, for example, dimethyl ether polyethylene glycol or N-methyl pyrrolidone.
These methods generally involve a stage of extraction of the H2S contained in the gas to be treated by contacting this gas with the regenerated solvent in an absorber operating at the pressure of the gas to be treated, followed by a thermal regeneration stage, generally at a pressure slightly higher than the atmospheric pressure, generally between 1 and 5 bara, preferably between 1.5 and 3 bara. This thermal regeneration is generally carried out in a column equipped in the bottom with a reboiler and at the top with a condenser allowing to cool the acid compounds released by the regeneration and to recycle the condensates to the top of the regenerator as reflux.
When the pressure of the gas to be treated is notably higher than the atmospheric pressure, for example in the case of a natural gas that has to be treated at a pressure of the order of 70 bar, the H2S-rich solvent obtained at the absorber bottom can contain significant amounts of dissolved hydrocarbons. It is then common practice to carry out a stage of release of these dissolved hydrocarbons vaporized by simple expansion of the H2S-rich solvent. This expansion is carried out at an intermediate pressure between that of the raw gas to be treated and that of the thermal regeneration stage, typically of the order of 5 to 15 bara. A gas containing the major part of the dissolved hydrocarbons, that can be used as fuel gas, is thus separated from the H2S-rich solvent. This gas is sometimes washed by a stream of regenerated solvent coming from the thermal stage so as to re-absorb the acid compounds, notably the H2S released upon expansion. This washing of the fuel gas released by expansion is generally performed in a column placed directly on the separator drum between the gas and the expanded liquid. The solvent thus laden with H2S is directly mixed with the expanded solvent and sent to the thermal regeneration stage.
In order to reduce the heat consumptions of these methods, a stage of thermal exchange between the rich solvent after expansion and the regenerated solvent obtained hot at the bottom of the regeneration column is generally carried out.
Regeneration of these solvents produces a gaseous effluent rich in acid compounds, essentially containing the extracted H2S and CO2. This acid effluent is generally subjected to a treatment in order to convert the H2S to elementary sulfur, non-toxic and easy to transport. The most commonly used conversion method is the Claus process, notably described in documents FR-2,494,255 and FR-2,327,960, wherein the acid gas extracted undergoes partial combustion in air or oxygen-enriched air generating a stoichiometric mixture of H2S and CO2 and of the elementary sulfur, recovered by condensation. This first thermal stage is generally followed by one to three catalytic conversion stages during which the H2S and the CO2 react and form elementary sulfur according to the Claus reaction:2 H2S+SO2←→3/x Sx+2 H2O
After the catalytic stages of the Claus process, a gas still containing notable amounts of sulfur products (SO2, H2S, as well as COS, CS2 and elementary sulfur) is obtained. In order to limit discharge of these compounds into the environment, this type of gas is generally subjected to a complementary finishing treatment. Various technologies have been proposed and used to carry out this type of finishing treatment. One of the most commonly used methods consists in converting all of the sulfur compounds of this gas to H2S, by reaction with reducing gases (hydrogen, CO) in the presence of a suitable catalyst. The residue gas thus obtained after this catalytic reduction stage is then washed by a solvent allowing selective extraction of the H2S and, after regeneration of this solvent, recycling of the H2S thus extracted to the thermal stage of the Claus plant.
It is possible to use a selective solvent, for example an aqueous MDEA solution, for washing the residue gas from the catalytic reduction stage downstream from the Claus plant.
When the raw gas to be treated is a natural gas containing CO2 and notable amounts of aromatic hydrocarbons (for example some hundred ppmv), notable amounts of these compounds are found in admixture with the H2S in the acid gas. In fact, although the stage of expansion of the H2S-rich solvent obtained at the bottom of the absorber allows to release the major part of the light hydrocarbons (methane, ethane, . . . ) dissolved in the solvent at the absorber bottom, it does not allow to extract the major part of the heavier compounds, in particular the aromatic compounds whose solubility in solvents is generally much higher than that of the aliphatic hydrocarbons. An acid gas that can contain several hundred ppmv of aromatic hydrocarbons is then commonly obtained at the regenerator top. Besides, even with solvents allowing selective absorption of H2S over CO2, a certain CO2 co-absorption is always observed. When the raw gas to be treated contains more CO2 than H2S, this co-absorption can lead to an acid gas containing large or even major proportions of CO2.
The simultaneous presence of large amounts of CO2 and of notable proportions (some hundred ppmv) of aromatic hydrocarbons in an acid gas leads to certain difficulties for conversion of the H2S of this gas to sulfur by means of the Claus process. In fact, dilution of H2S by CO2 reduces the temperature obtained in the oven of the Claus thermal stage. This temperature reduction in turn decreases the destruction of the aromatic compounds, which are then present in notable proportions in the subsequent catalytic stages. The presence of these aromatic compounds during these catalytic stages can then cause various operating problems such as: production of coloured sulfur contaminated by carbon-containing compounds and therefore unfit for sale, clogging and activity loss of the catalysts by formation and deposition of carbon-containing compounds on the catalysts (carsuls).
Furthermore, when dilution of the H2S in the gaseous effluent produced during regeneration becomes too high, it is no longer possible to have a thermal stage in the Claus process. One may then consider treating highly diluted gases (containing only some % by volume of H2S) by means of direct oxidation processes wherein the acid gas and the air are directly contacted in the presence of a suitable catalyst allowing the reaction between the H2S and the oxygen of the air to be controlled so as to essentially produce only sulfur, but then again the presence of a large proportion of aromatic hydrocarbons makes it difficult to use these catalysts.
Various solutions have been proposed to overcome these drawbacks, among which:                preheating the gases (acid gas and air) feeding the burner of the thermal stage of the Claus process. Such a preheating operation allows the temperature to be increased in the oven of the Claus plant. Depending on the CO2 content of the gas, this solution does however not always allow to reach the temperature levels required to obtain nearly-total destruction of the aromatic hydrocarbons (of the order of 1150° C. or more), unless expensive preheating methods are implemented,        absorption of the aromatic compounds present in the acid gas on a suitable material (activated charcoal for example). This method requires an additional processing unit that may be expensive as regards investment (case of a regenerable adsorbent) or operating costs (case of non-regenerable adsorbents),        acid gas enrichment by selective re-absorption of the H2S it contains in a suitable solvent. This method is very efficient as regards sulfur production since it allows to obtain, on the one hand, an H2S-depleted gas containing most of the CO2 and of the aromatic hydrocarbons present in the acid and, on the other hand, an H2S-concentrated gas depleted in aromatic hydrocarbons. It however represents a significant investment since all of the H2S extracted from the raw gas to be treated has to be re-absorbed.        
The present invention provides a simple and inexpensive method that requires only a small number of additional equipments for separating the major part of the aromatic hydrocarbons co-absorbed by the solvent from the major part of the hydrogen sulfide absorbed by the solvent.